1. Field of the Invention
The present invention relates to a method of manufacturing a stator for a stepping motor and more particularly to a method of manufacturing a stator for a micro stepping motor whose outer cylindrical section has the diameter of 30 mm or less or a stator for a small size stepping motor having an outer cylindrical section drawn deeply.
2. Related Background Art
Generally, a stepping motor has, as shown in FIG. 1, a rotor 1, a shaft 2 and stators 3.sub.1, 3.sub.2, 4.sub.1 and 4.sub.2. The stators 3.sub.1, 3.sub.2, 4.sub.1 and 4.sub.2 are each formed with a plurality of magnetic pole teeth (comb teeth) 3a, 3b, 4a or 4b at regular intervals in the peripheral direction. It is preferable to form the length of the comb teeth 3a, 3b, 4a and 4b long in order to increase the effective magnetic flux generated by the excitation of excitation coils 5a and 5b. However, when the comb teeth are formed by simply blanking the center of a single frame plate as is conventional, the length of the comb teeth becomes less than half the blanking diameter, limiting the ability to obtain large effective magnetic flux. Although Japanese Patent Publication No. 57-211964 discloses a method for solving the above problem, a study was made to investigate a further problem upon manufacturing and a method was proposed in a United States Patent Application filed on Mar. 1, 1994.
However, when the stepping motor is formed to be a micromotor or the outer cylindrical section of the stepping motor is manufactured by a drawing process, the above methods are not used very much, since the outer cylindrical section is shaped so as to be elongated to a great extent as shown in FIGS. 3 to 5. Such a method wherein the cylindrical section is so elongated has been proposed in Japanese Patent Publication No. 3-53854.
In this method, as shown in cross section in FIG. 12A, first, a frame plate 10 is subjected to a drawing process to form a cylindrical section 14 on the outer periphery of a flat section 12 such that the diameter of the flat section 12 becomes D.sub.1. Next, as shown in FIG. 12B, a plurality of comb teeth 16 are cut from the flat section 12 and raised from the flat section 12 at a right angle such that the inside diameter of a concentric circle drawn by the root portions of the comb teeth becomes d.sub.1 and tips of the comb teeth are located close to the outer periphery of the flat portion 12. Next, as shown in FIG. 12C, the cylindrical section 14 is further subjected to a drawing process to make the diameter of the flat section 12 becomes D.sub.2. Finally, as shown in FIG. 12D, the cylindrical section 14 is subjected to a final drawing process to make the diameter of the flat section 12 be D.sub.3 and simultaneously the lower portion of the cylindrical section 14 is bent at a right angle thereby to complete formation of a stator. According to this method, it is possible to make the length of the comb teeth equal to or more than half the value obtained by subtracting the inside dimaeter d.sub.1 of the concentric circle of the comb teeth from the diameter D.sub.3 of the flat portion 12.
However, there are following problems in the above conventional method.
(1) The cutting and raising process is performed such that while the plate is supported by a die from underneath, a punch is lowered to punch the plate to form the comb teeth and the punch is further lowered until the comb teeth are brought into contact with the peripheral surface of the die to be raised at a right angle. Also, the punch is formed such that it is first brought into contact with a portion of the flat section 12 closer to the cylindrical section 14 and thereafter is gradually brought into contact with inner portions of the flat portion 12. Therefore, the force applied to the comb teeth during the cutting and raising process is shifted gradually from the tips of the comb teeth to the root portions thereof. As a result, distortions occur in the comb teeth. Then, after the cutting and raising process, cambers are produced on the comb teeth 16, as indicated by the broken line in FIG. 13, which are different from an ideally straight shape, as indicated by the solid line in FIG. 13. The occurrence of the cambers causes the power of a motor in which such a stator is used to be lowered. Especially, when there are variations in the shapes of the cambers of the comb teeth, the power of the motor is further lowered. PA1 (2) Also, in the cutting and raising process, if there is a problem in the intensity of the die when the cutting and raising of all the comb teeth are performed, the cutting and raising are performed two or three times. In this case, after the first cutting and raising process, spaces are formed in the flat portion, where some comb teeth are cut and raised. Then, in the second cutting and raising process, a force (in the peripheral direction) directed to the spaces is generated, which causes comb teeth for the second cutting and raising process to be distorted in the peripheral direction, as indicated by the broken line in FIG. 14, so that their shape is different from the ideal shape as indicated by the solid line in FIG. 14. That is, the the comb teeth are inclined in the peripheral direction. When the resultant stator constitutes a stepping motor, such inclination of the comb teeth in the peripheral direction causes variations in calculating the step angles, resulting in the decrease in the positioning accuracy. PA1 (3) Also, although a surface treated steel sheet is utilized as the material of the plate 10, there are variations in thickness of the surface treated steel sheet due to the rolling characteristics thereof. For example, usually, there is about a 4% maximum difference in thickness in the width and rolling directions of the sheet (FIG. 15). When such a plate with variations in thickness is subjected to the cutting and raising process to form the comb teeth, the squareness or right angle accuracy in forming a right angle with the comb teeth with respect to flat section 12 is lowered, reducing the power of any motor made from a stator using such a sheet. PA1 (4) When the drawing process is conducted after the cutting and raising process, a force is applied to the raised comb teeth during the drawing process, which causes the comb teeth to be distorted in the radial or peripheral direction.